[RFC] Option to set single-click as default and sigle-click handling in system settings' main window

Fixes lxde/lxde-qt#266.

The single-click handing in System Settings' main window is a temporary fix until a more clean approach is found. See the discussion (lxde/lxde-qt#266). It might as well not be very clear that the option does not work for pcmanfm-qt, which has it's own configuration options.